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Thomas Aquinas On Being
and Essence
Prologue
Because a small error in the beginning grows enormous at the end, as the Philosopher
remarks in Book 1 of On the Heavens and the World,1 and being and essence are the first
things to be conceived by our understanding, as Avicenna declares in Book 1 of his
Metaphysics,2 in order to avoid falling into error about them, and to reveal their difficulties,
we should see what are signified by the names of being and essence, how these are found
in various things, and how they are related to the logical intentions3 of genus, species, and
difference.
And since we need to arrive at the cognition of simple components from the cognition
of what they compose, and from those that are posterior to those that are prior, so that
the discussion may suitably progress from the easier subjects, we should proceed from the
signification of the name of being to the signification of the name of essence.4
1 Aristotle, De Caelo, lb. 1, c. 5, A271; Commentary of St. Thomas, lc. 9.
2 Avicenna, Metaphysica, lb. 1, c. 6.
3 The “logical intentions” are second-order concepts (secundae intentiones) whereby we conceive of
first-order concepts [primae intentiones], which in turn are concepts whereby we conceive of things that
are not concepts. For example, by means of the first-order concepts of humans and of dogs we conceive of human beings and of dogs, which are not concepts. But by means of the second-order concept of species we conceive of the first-order, specific concepts of humans, dogs, donkeys, etc., and by
means of the second-order concept of genus we conceive of the generic concepts of animals, plants,
elements, etc.
4 This passage is a neat expression of Aquinas’s general Aristotelian epistemological stance. We should
start with the things with which we are more familiar, namely, the things given in our everyday experience, which are, nevertheless, metaphysically posterior to their simple components. But through the
careful conceptual analysis of these things we may arrive at the understanding of their simple metaphysical constituents, which will then enable us to see how these constituents can be found even in
things beyond our everyday experience.
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Chapter 1. The Meanings of the Names of Being and Essence5
We should know that, as the Philosopher says in Book 5 of the Metaphysics, something is
said to be a being [ens per se] 6 in two different senses: in one sense, [only] those things [are
called beings] that are sorted into the ten categories; in the other sense [calling something a
being] signifies the truth of a proposition. And the difference between the two is that in the
second sense everything can be said to be a being of which a [true] affirmative proposition
can be formed, even if it posits nothing in reality; it is in this way that privations and negations are said to be beings, for we say that an affirmation is the opposite of negation, and
that there is blindness in an eye. But in the first sense only that can be said to be a being
which posits something in reality.7 Therefore, blindness and the like are not beings in the
first sense. So, the name “essence” [essentia] is not derived from “being” [ens] in the second
sense (for some things are said to be beings in this sense although they do not have an essence,
as is clear in the case of privations), but from “being” as it is used in the first sense. This is
why the Commentator remarks in the same passage: “ ‘Being’ in the first sense signifies the
substance of the thing.” 8
And since, as has been said, beings in this sense are sorted into the ten categories;
“essence” has to signify something that is common to all natures on account of which
various beings fall under the diverse genera and species, as for example humanity is the essence
of man, and so on for the rest. And because that by which a thing is constituted in its proper
genus or species is what we signify by the definition indicating what [quid] this thing is, philosophers changed the name of essence into the name “quiddity” [quidditas], and this is what
the Philosopher in Book 7 of the Metaphysics often calls “what something was to be” [quod
5 The division of the text into seven chapters follows the Boyer edition, used for this translation
(“Opusculum De ente et essentia, introductione et notis auctum,” ed. C. Boyer, Textus et Documenta,
Series Philosophica, 5: Pontificia Universitas Gregoriana (Rome, 1933)). The chapter titles are added
by the translator.
6 Aristotle, Metaphysics, Bk. 5, c. 7, 1017a22–3; Commentary of St. Thomas, lc. 7. In the passage to
which Aquinas refers, Aristotle contrasts “coincidental being” (ens per accidens) with “non-coincidental
being” [ens per se]. A coincidental being is something identified on the basis of merely coincidental features, on account of which it would belong to two different categories; for example, a tall musician,
described as such, is a merely coincidental being, described in terms of a quantity (height) and a quality (a capacity to play music). So, the person conceived and described in this way is a merely coincidental being, an ens per accidens. On the other hand, the same person considered merely as a human
being, as such, is not described in coincidental terms, so, the person considered in this way is an ens
per se. Obviously, the distinction itself is not a real distinction between two different sorts of things,
but a mere distinction of reason between different ways of conceiving of the same thing.
7 This, initially puzzling, distinction between the two main senses of the term “being” is what is often
referred to as the distinction between “real being” (ens reale) and “being of reason” (ens rationis) (sometimes also referred to as “being as true” (ens ut verum)). The distinction will at once be less puzzling,
if it is read in its proper context, i.e., the “inherence theory of predication” and the corresponding
theory of signification, assumed here by Aquinas, without any preliminary explanation. For detailed
analyses of this distinction along these lines, see G. Klima, “The Semantic Principles Underlying Saint
Thomas Aquinas’s Metaphysics of Being,” Medieval Philosophy and Theology, 5 (1996), pp. 87–141;
“Aquinas’ Theory of the Copula and the Analogy of Being,” Logical Analysis and History of Philosophy,
5 (2002), pp. 159 –76; “The Changing Role of Entia Rationis in Medieval Philosophy: A Comparative
Study with a Reconstruction,” Synthese 96 (1993), pp. 25–59.
8 Averroës, In Metaph., lb. 5, com. 14.
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quid erat esse], that is to say, that on account of which something is what it is. It is also called
“form,” because “form” signifies the perfection and determinate character [certitudo] of every
thing, as Avicenna says in Book 2 of his Metaphysics.9 It is also called “nature,” taking
“nature” in the first of the four senses assigned to it by Boethius in his On Two Natures,10
namely, in the sense in which a nature is said to be that which can in any way be apprehended by the intellect. For a thing is intelligible only on account of its essence and
definition; and it is also in this sense that the Philosopher in Book 5 of the Metaphysics says
that every substance is a nature. But the name “nature” taken in this way seems to signify
the essence of a thing insofar as it is related to the thing’s proper operation, since nothing
is deprived of its proper operation. The name “quiddity,” on the other hand, is derived from
the fact that it is signified by the definition; but it is called “essence” [essentia], because it is
that through which and in which a thing has its being [esse].
Chapter 2. The Essence of Material Substances
But because a substance is said to be a being primarily and without qualification, whereas
an accident is a being only secondarily and, as it were, with qualification, only a substance
has an essence in a strict and true sense, while an accident has it only somehow, with
qualification.
Some substances are simple, others are composite, and both sorts have their essence, but
the simple ones in a more genuine and excellent way, just as they have a more excellent
way of being. For they are the cause of the composite ones; at least this is true of the first,
simple substance, which is God.
However, since the essences of these substances are quite hidden from us, we should
begin with the essences of composite substances, so that our discussion may more suitably
proceed from the easier subjects.
In composite substances, therefore, form and matter are known [components],11 as are
soul and body in man. But it cannot be said that one of these alone should be called the
essence of the thing. That matter alone is not the essence of the thing is clear, because
the thing is knowable through its essence, and it is on account of its essence that it falls
under its proper species and genus; but its matter is not the principle of cognition of the
thing, and it is not on account of its matter that a thing is determined to be in its proper
genus or species, but that on account of which it actually exists. However, the form of a
thing alone cannot be said to be its essence either, even if some people want to make this
claim.12 For on the basis of what has been said so far it is clear that the essence of a thing
is what its definition signifies. But the definition of things of nature contains not only
form, but matter as well; otherwise natural definitions would not differ from mathematical
9 Avicenna, Metaph., 1, 6; 2, 2.
10 De persona et duabus naturis, c. 1. Migne Patrologia Latina (ML), 64, 1341.
11 Matter and form are the obviously given metaphysical components of any material substance, as
it is clear from the hylomorphic analysis of physical change, provided by Aquinas in his On the Principles
of Nature.
12 In particular, Averroës and his followers made this claim against Avicenna. Aquinas sides with
Avicenna on the issue. Cf. Aquinas, In Metaphysica, lb. 7, lc. 9, n. 1467; In Quattuor Libros Sententiarum
Petri Lombardi (SN ) 4, d. 44, q. 1, a. 1b, resp. 2.
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definitions.13 And it cannot be said that matter is included in the definition of a natural
substance as something added to its essence, or as something existing outside of its essence,
for this sort of definition is proper to accidents, which do not have perfect essence, and therefore they have to receive in their definition their subject, which is outside their essence.14
It is clear, therefore, that essence comprises both matter and form.
Nevertheless, it cannot be said that “essence” signifies a relation between matter and form,
or something superadded to these, because that would necessarily be an accident or extrinsic
to the thing, and the thing would not be known on account of it, none of which can apply
to an essence. For by the form, which is the actuality of matter, matter is made into an actual
being and this [particular] thing [hoc aliquid]. Therefore, whatever supervenes does not give
actual being to matter absolutely speaking, but it makes it actual in some respect, just as
accidents do, as whiteness makes something actually white. So, when a form of this sort is
acquired, the thing is not said to be generated absolutely speaking, but only in some respect.
What remains, therefore, is that the name “essence” in composite substances signifies what
is composed of matter and form. And Boethius agrees with this position in his Commentary
on the Categories, when he says that ousia signifies something composite; for ousia signifies
the same for the Greeks as “essence” does for us, as he says in his On Two Natures. Avicenna
also says that the quiddity of composite substances is the composition of matter and form
itself.15 And even Averroës declares in his Commentary on Book 7 of the Metaphysics: “the
nature that the species of generable and corruptible things have is a certain intermediary,
that is, composed of matter and form.”16
And reason agrees with this as well, for the act of being [esse] of a composite substance is
neither of the form alone, nor of matter alone, but of the composite itself; and the essence
is that on account of which the thing is said to be. Therefore, the essence, on account of
which the thing is denominated a “being,” cannot be the form alone or the matter alone,
but has to be both, although it is the form that causes this act of being in its own way. We
can see also in cases of other things composed of several principles that the thing is not denominated on account of only one of those principles that constitute it, but on account of what
comprises both, as is clear in the case of flavors: for sweetness is caused by the action of
warmth spreading wetness, and although in this way warmth causes sweetness, still, a body
is not denominated sweet on account of its warmth, but on account of its flavor, which comprises both warmth and wetness.17
13 For example, the geometrical definition of a sphere would not differ from the natural definition
of a pearl. But even if all pearls are essentially spherical (more or less), they also essentially consist of
layers of crystallized calcium carbonate held together by conchiolin, which is why not all spherical bodies are pearls.
14 This point is discussed in more detail in c. 7 below, considering the definition of “pugness,” which
is necessarily a curvature of a nose, but in and of itself is the curvature only, having to have a nose as
its subject. Cf. also n. 67 below.
15 Avicenna, Metaph., 5, 5.
16 Averroës, In Metaph., 7, 7, com. 27.
17 The example, based on the somewhat obscure Aristotelian doctrine of how the composite qualities of mixed bodies can be derived from the interaction of the simple active and passive qualities of
the four elements, is not very illuminating to us. But the point is clear, and can, in fact, be illustrated
by another, more familiar example: punch flavor is caused by mixing various fruit juices (and maybe
some rum). Still, a punch-flavored cake or scoop of ice cream is not denominated by any of the components, but by the composite flavor resulting from the mixture.
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However, since the principle of individuation is matter, from this it may appear to
follow that the essence, which comprises both matter and form, is only particular and not
universal, from which it would further follow that universals would not have definitions, if
an essence is something that the definition signifies.
And for this reason we should know that matter considered in just any way is not the
principle of individuation, but only designated matter is. And by designated matter I mean
matter considered under determinate dimensions. This matter is not included in the
definition of man as such, but it would be included in Socrates’ definition, if Socrates had a
definition. The definition of man, on the other hand, includes non-designated matter; for the
definition of man does not include this bone or this flesh, but bones and flesh absolutely,
which are the non-designated matter of man.18
Chapter 3. The Composition of Material Substances
In this way it is clear that the essence of man and the essence of Socrates differ only as designated and non-designated, and this is why the Commentator declares in his Commentary
on Book 7 of the Metaphysics: “Socrates is nothing other than animality and rationality, which
are his quiddity.”19 It is also in the same way that the essence of the genus and the essence
of the species differ as designated and non-designated, although the manner of designation
is different in the two cases. For the designation of the individual with respect to the species
takes place through matter determined by dimensions, whereas the designation of the
species regarding the genus occurs through a constitutive difference, which is taken from
the form of the thing.
But the determination or designation of the species with respect to the genus is not through
anything in the essence of the species that would in no way be in the essence of the genus;
rather, whatever is in the species is also in the genus indeterminately. For if an animal were
not the same as the whole that is a man, but only a part of it, then animal would not be
predicated of man, for no integral part is predicated of its whole.
We can see how this happens if we consider how body, insofar as it is taken to be a part
of an animal, differs from body, insofar as it is taken to be a genus; for it cannot be a genus
in the sense in which it is an integral part. The name “body,” therefore, can be taken in
several senses. For a body, insofar as it is in the genus of substance, is said to be a body
because it has a nature on account of which three dimensions can be designated in it; while
18 It is important to note here that the difference between designated and non-designated matter
is, again, not a difference between two different kinds of entities, but rather a difference between two
different ways of considering the same kind of entity; that is to say, the distinction between designated
and non-designated matter is not a real distinction, but a distinction of reason. All matter in reality is
designated matter, i.e., concrete chunks of matter existing under the determinate dimensions of the
individual bodies they constitute. But just as the individuals can be considered in a universal manner,
disregarding their individuating features, so their matter can be considered generally, not as this or
that particular chunk of matter. And it is obvious that it is only matter considered in this way that
can be signified by the definition of the species (the definition that signifies the specific essence of each
individual), for it is clear that the definition of man, for example, cannot signify the flesh and bones
of Socrates as this particular matter belonging to Socrates, for otherwise all men (i.e., all individuals
having the essence signified by this definition) ought to have Socrates’ flesh and bones, which is clearly
absurd. Cf. n. 26 below.
19 Averroës, In Metaph., 7, c. 5, com. 20.
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the three designated dimensions themselves are the body which is in the genus of quantity.
But it happens that something that has some perfection also reaches a further perfection, as
is obvious in the case of man, who has a sensitive nature and beyond that also an intellective one. Likewise, to the perfection of a thing’s having a form on account of which three
dimensions can be designated in it, a further perfection can be added, such as life, or something like that. The name “body,” therefore, can signify something which has a form from
which there follows the designatability of three dimensions exclusively, namely, in such way
that from that form no further perfection would follow, but if something is added, then it
is beyond the signification of “body” in this sense. And in this sense the body will be
an integral and material part of an animal, for in this way the soul will be beyond the signification
of the name “body,” and it will be superadded to the body itself, so that the animal will be
constituted from these two, namely, from the soul and the body, as its parts.
But the name “body” can also be taken to signify some thing that has a form on account
of which three dimensions can be designated in it, whatever that form may be, whether it
may give rise to some further perfection or not. And in this sense body will be a genus of
animal, because animal contains nothing that is not contained implicitly in body. For the soul
is not another form than that on account of which in that thing three dimensions can be
designated;20 and so when it was said that a body is something that has a form on account
of which three dimensions can be designated in it, this form was understood to be any form,
regardless of whether it is animality or lapideity [rock-ness] or anything else of this sort.
And in this way the form of animal is implicitly contained in the form of body, insofar as
body is its genus.
There is a similar relationship between man and animal. For if “animal” only named a thing
that has the perfection of sensing and moving on account of an intrinsic principle with the
exclusion of any further perfection, then any further supervening perfection would be related
to animal as to a part, and not as implicitly contained in the notion of animal. And in this
way animal would not be a genus. But it is a genus insofar as it signifies a certain thing whose
form can give rise to sensation and motion, no matter what sort of form it is, whether it is
a sensitive soul only or a soul that is sensitive and rational as well. In this way, therefore,
the genus indeterminately signifies the whole that is in the species, for it does not signify
only matter.
Likewise, the difference signifies the whole and not only form; and even the definition
signifies the whole, as does the species, but in different ways. For the genus signifies the whole
as a certain denomination determining what is material in the thing without determining its
proper form; and the genus is taken from matter, although it is not matter. Hence it is clear
that something is said to be a body because it has the perfection that three dimensions can
be designated in it, which is a perfection that is related as something material to the further
perfections. The difference, on the other hand, is a certain determination taken from a determinate form without having some determinate matter in its primary notion, as is clear in
the case of animate, namely, something that has a soul [anima], for this does not determine
20 This is an important expression of Aquinas’s famous, and in his time very controversial, thesis of
the unity of substantial forms. For a detailed discussion of this passage, see G. Klima, “Man = Body +
Soul: Aquinas’s Arithmetic of Human Nature,” in B. Davies (ed.), Thomas Aquinas: Contemporary
Philosophical Perspectives, Oxford: Oxford University Press, pp. 257–73. For more discussion of the thesis and its history, see D. A. Callus, “Forms, Unicity and Plurality of,” in New Catholic Encyclopedia,
New York: McGraw-Hill; 1967–79.
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what sort of thing it is, whether a body or something else. This is why Avicenna says21 that
the genus is not understood in the difference as a part of its essence, just as their subject is
not part of the understanding of attributes [ passiones]; and so the genus is not predicated of
the difference per se, as the Philosopher remarks in Book 3 of the Metaphysics and Book 4
of the Topics,22 except, perhaps, in the way the subject is predicated of the attribute. But the
definition and the species comprise both, namely, the determinate matter designated by
the name of the genus and the determinate form designated by the name of the difference.
And from this it is clear why the genus, difference, and species are analogous to matter,
form, and the composite in nature, although they are not identical with them. For the genus
is not matter, but is taken from matter as signifying the whole, and the difference is not form,
but is taken from the form as signifying the whole. Therefore, we say that man is a rational
animal, but not that man is composed of animal and rational, as we say that man is composed
of body and soul. For man is said to be composed of soul and body, as a third thing constituted by two things, neither of which is identical with it. For a man is neither soul nor
body. But if man is somehow said to consist of animal and rational, then it will not be a third
thing constituted by two other things, but rather a third concept constituted by two other
concepts. For the concept of animal does not determine the specific form expressing the nature
of the thing, because it is material with respect to the ultimate perfection; and the concept of
the difference rational consists in the determination of the specific form; and the concept
of the definition or species is constituted by these two concepts. Therefore, just as a thing
constituted by others cannot be said to be any of those that constitute it, so a concept
cannot be said to be any of the concepts that constitute it; for we do not say that the definition
is the genus or the difference.
Although the genus signifies the entire essence of the species, still, the several species in
the same genus do not have to have the same essence; for the unity of the genus stems from
its indetermination or indifference. However, this does not happen in such a manner that
what the genus signifies is some numerically one nature in the diverse species having some
other thing supervening on it, namely, the difference determining it, as the form determines
matter that is numerically one; rather, this happens because the genus signifies some form,
but not determinately this or that form, which in turn is determinately expressed by the
difference; however, this [determinately expressed form] is nothing else but what was
indeterminately signified by the genus.23 And this is why the Commentator says in his
Commentary on Book 12 of the Metaphysics that prime matter is said to be one on account
of the removal of all forms, but the genus is one on account of the community of the form
signified.24 So, it is clear that the addition of the difference, which removes the indetermination that was the cause of the unity of the genus, yields essentially diverse species.
And since, as has been said, the nature of the species is indeterminate with respect to the
individual, just as the nature of the genus is with respect to the species, therefore, just as
21 Avicenna, Metaph., 5, 6.
22 Aristotle, Metaph., 3, 3, 998b24; St. Thomas’s Commentary, lc. 7; Topics, 4, 2, 122b20; 6, 6, 144a32.
23 So, “animal” indeterminately signifies the animality of Socrates and the animality of his dog. But
Socrates’ animality is nothing else but his humanity, which is determinately signified by the difference
“rational,” and his dog’s animality is nothing else but his “caninity” (dogginess), which is determinately
signified by the proper specific difference of dogs (whatever that is).
24 Averroës, In Metaph., 12, com 14.
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the genus, insofar as it is predicated of the species, contains25 in its signification everything
that is determinately in the species, although indistinctly, so the species, insofar as it is
predicated of the individual, has to signify everything that is essentially in the individual, although
indistinctly; and this is how the essence of the species is signified by the name “man”;
therefore, “man” is predicated of Socrates. However, if the nature of the species is signified
with the exclusion of designated matter, which is the principle of individuation, then it is
related to the individual as its part, and this is how it is signified by the name “humanity”;
for “humanity” signifies that on account of which a man is a man. But designated matter is
not that on account of which a man is a man, and so it is not contained in any way among
the things from which a man has it that he is a man. Since, therefore, “humanity” includes in
its understanding only those things from which a man has it that he is a man, it is clear that
designated matter is excluded or cut off from its signification. And since a part is not predicated of the whole, this is why “humanity” is not predicated of a man or of Socrates.26 This is
why Avicenna remarks that the quiddity of the composite [substance] is not the composite
[substance], although the quiddity itself is composite; just as humanity, even if it is composite,
nevertheless, is not a man, but it has to be received in something, which is designated matter.
But, as has been said, the designation of the species regarding the genus is by means
of forms, whereas the designation of the individual regarding the species is by means of
25 We get this translation by amending the text with the addition of the obviously missing verb
“continet.”
26 That is to say, the propositions: “A man is a humanity” and “Socrates is a humanity” are false.
This is an expression of Aquinas’ metaphysical claim that a material substance cannot be identical with
its essence, but its essence is its part that is really distinct both from the whole and from the other part
of the same individual, namely, its designated matter. Note that the real distinction between the specific
essence and the designated matter of the same individual is not the same as the real distinction between
the matter and substantial form of the same individual, although both distinctions are based on the
materiality of the individual in question. These are two different ways of conceptually “carving up”
the same individual. (This does not have to mean, however, that the parts thus distinguished are not
really distinct: just because we can conceptually distinguish the left half and the right half, as well as
the top half and the bottom half of a body, it does not mean that these parts are not really distinct
regardless of whether we distinguish them or not.) The (prime) matter and substantial form of the
same individual are distinguished on account of the analysis of substantial change: the prime matter
of a thing is what takes on a new substantial form when the thing ceases to be, and a new thing comes
to be from it, say, when hydrogen turns into helium in a nuclear fusion, or when a living being dies
and turns into a corpse. Therefore, the form so distinguished excludes this matter from its notion. But
the essence signified by the definition, as we could see above, does not exclude matter in general,
but only this designated matter in particular (see n. 18 above), and only when it is signified exclusively,
by the abstract form of the name of the species, as by the name “humanity.” So, this essence, as signified
by this term, is a formal component of the whole substance, received in, and individuated by, the
designated matter of this substance. But this formal component is not exactly the same as the
substantial form which is received in prime matter, for this formal component comprises both matter
and form considered in general, excluding only what individuates them, namely, this designated
matter, which is why this formal component, the essence, is also referred to as “the form of the
whole” ( forma totius), to distinguish it from “the form of the part” ( forma partis), which is the form
referred to in the other division, distinguishing substantial form and prime matter. So, an individual
material substance, say, Socrates (disregarding his accidents) = substantial form ( forma partis) + prime
matter = essence ( forma totius) + designated matter. Besides these two divisions of Socrates, there is
a third one, not coinciding with either of these two, according to which Socrates = body + soul.
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matter; therefore, the name signifying that from which the nature of the genus is taken with
the exclusion of the determinate form that perfects the species has to signify the material
part of the whole, as for example the body is the material part of man; while the name
signifying that from which the nature of the species is taken, with the exclusion of designated
matter, has to signify the formal part. So, humanity is signified as a sort of form, and is said
to be the form of the whole [ forma totius], but not, as it were, as something added over and
above the essential parts, namely, form and matter, as the form of a house is added over
and above its integral parts, but rather it is a form that is a whole, namely, comprising
both matter and form, however, with the exclusion of those things that are apt to designate matter.27
In this way it is clear that the essence of man is signified both by the name “man” and by
the name “humanity,” but differently, as has been said. For the name “man” signifies it as
a whole, namely, insofar as it does not exclude the designation of matter, but contains it
implicitly and indistinctly, just as we said the genus contains the difference. And this is why
the name “man” is predicated of the individuals. But the name “humanity” signifies it as
a part, because it contains in its signification only what pertains to a man insofar as he is a
man, and excludes all designation of matter; therefore it is not predicated of individual humans.
And for this reason, the name “essence” is sometimes found to be predicated of a thing, for
it is said that Socrates28 is a certain essence; and sometimes it is negated, when we say that
Socrates’ essence is not Socrates.
Chapter 4. The Essence of Material Substance and
the Logical Intentions
Having seen what is signified by the name “essence” in material substances, we should
see how it is related to the notions of genus, species, and difference. Now, since that to
which the notion of genus, species, or difference applies is predicated of this designated
individual,29 the notion of a universal, that is, of genus or species, cannot apply to essence
insofar as it is signified as a part, for example, by the name “humanity” or “animality.”
And this is why Avicenna says that rationality is not a difference, but the principle of a
difference;30 and for the same reason, humanity is not a species, nor is animality a genus.
Likewise, one cannot say that the notion of genus or species applies to essence insofar as it
is something existing outside the singulars, as the Platonists believed; for in this way the genus
and species would not be predicated of this individual; for it cannot be said that Socrates is
something that is separate from him; and neither would that separate thing be of any use in
the cognition of this singular. Therefore, it remains that the notion of genus or species applies
to essence insofar as it is signified as a whole, for example, by the names “man” or “animal,”
as it implicitly and indistinctly contains all that is in the individual.
But a nature or essence so taken can be considered in two ways. In one way, according
to its proper notion; and this is its absolute consideration. And in this way nothing is true
27 “Those things that are apt to designate matter” are the dimensions of the material substance, whereby
it occupies its location in space, as distinct from other bodies.
28 Correcting the text’s “Socratis” to “Socrates.”
29 This is because according to the “canonical” definitions of Porphyry in his Isagoge (which was often
referred to as Praedicabilia), all universals are “predicables,” i.e., items that can be predicated of several
individuals.
30 Avicenna, Metaph., 5, 6.
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of it except what pertains to it on account of what it is; therefore, if anything else is
attributed to it, then the attribution is false. For example, to a man, on account of being a
man, it pertains to be rational and animal and anything else that is in his definition; to be
white or black, however, and anything else that is not involved in the notion of humanity
does not pertain to a man on account of being a man. Therefore, if it is asked whether this
nature considered in this way can be said to be one or many, neither alternative should be
accepted, because both are outside of the understanding of humanity, and either can pertain
to it. For if plurality were included in its understanding, then it could never be one, although
it is one insofar as it is in Socrates. Likewise, if unity were included in its notion and understanding, then Socrates and Plato would have [numerically] one and the same nature, and
it could not be multiplied in several things.31 In the other way nature is considered according to its existence in this or that thing. And in this way something can be predicated of it
accidentally [per accidens], on account of the thing in which it is. For example, it is said that
a man is white, because Socrates is white, although this [i.e., being white] does not pertain
to a man on account of being a man.
This nature has two sorts of existence [esse]: one in the singulars, another in the soul, and
accidents follow upon this nature on account of each. And in the singulars it has several acts
of existence [esse] according to the diversity of singulars. But the nature itself, according to
its proper, that is absolute, consideration, does not have to have any of these. For it is false
to say that the nature of man as such should have existence in this singular, for then it could
never exist outside this singular [i.e., in another singular]; likewise, if it pertained to [the nature
of] man as such not to exist in this singular, then it would never exist in this singular. But
it is true to say that to [the nature of] man as such it does not pertain that it should exist
in this singular or in that one or in the soul. It is clear, therefore, that the nature of man
absolutely considered abstracts from all existence, without, however, excluding any of
them.32 And it is the nature considered in this way that is predicated of all the individuals
[that have this nature].33
Nevertheless, one cannot say that the notion of universals applies to natures taken in this
way; for the concept of universals includes unity and community,34 but neither of these applies
to human nature according to its absolute consideration. For if community were understood
31 This is why it is useful to think of this “common nature,” i.e., the essence of any thing considered absolutely, as the common information content informing several individuals in its several
instances, just as the common information content of a book is multiplied in its several copies, or the
same genetic information is multiplied and reproduced in several generations of individual living things.
32 So the common nature as such is indifferent to any act of being and so to any mode of being,
which is why it can exist in any appropriate singular subject in different ways, i.e., as the common
information content of all these singulars, it can exist encoded in different media, whether in the soul
or in the actual individuals, just as the same song, for example, can be encoded in magnetic or optical
media, or as actual vibrations of the air, i.e., actual sounds.
33 Clearly, the predicate of the sentence “Socrates is a man” attributes to Socrates only what pertains to the nature signified by it according to its absolute consideration. For knowing that this sentence is true, we know that Socrates is a rational animal, and that he is a sensitive living body, etc.,
namely, whatever is involved in being a man as such, but we certainly do not know whether he is
black or white, short or tall, handsome or ugly, etc., anything that does not pertain to human nature
according to its actual consideration.
34 For according to one formulaic definition, universale est unum commune multis, a universal is [something] one common to many.
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to be implied in the concept of man, then there would have to be community in anything
in which there is humanity; but this is false, since there is no community in Socrates, but
whatever is in him is individuated. Similarly, it cannot be said that the notion of genus or
species applies to human nature insofar as it exists in individuals; for in the individuals human
nature does not have the sort of unity according to which it is some single thing pertaining
to all [of the same nature], which the notion of universals requires.
It remains, therefore, that the notion of species applies to human nature insofar as it exists
in the intellect. For human nature itself exists in the intellect abstracted from all individuating conditions, whence it is uniformly related to all individuals [of this nature] outside the
soul, being equally a similitude of all, and thus leading to the cognition of all, insofar as
they are humans. And since it has this sort of relation to all individuals [of this nature], the
intellect forms the notion of species and attributes to it,35 and this is why the Commentator
remarks in his commentary on Book 1 of On the Soul that it is the intellect that produces
universality in things;36 Avicenna also makes the same point in his Metaphysics.37
Now, although this nature as understood is universal, insofar as it is compared to the things
outside the soul, for it is one similitude of them all, nevertheless, insofar as it exists in this
or in that intellect, it is some particular species actually understood [species intellecta]. And
from this it is easy to see the error of the Commentator who, in his commentary on Book
3 of On the Soul, on the basis of the universality of the form understood, wanted to conclude
that there is a single intellect for all human beings; for that form is not universal in its
existence that it has in the intellect, but insofar as it is a similitude of things, just as if there
were a corporeal statue representing several people, it is obvious that the image or shape
of the statue would have its own singular act of existence [esse], insofar as it would exist
in this matter, but it would be common, insofar as it would be a common representation
of several things.
Since human nature is predicated of Socrates according to its absolute consideration, and
that it is a species does not pertain to it according to its absolute consideration, but it is one
of the accidents it acquires on account of its being in the intellect, the name “species” is not
predicated of Socrates, so that one could say “Socrates is a species.” But this would have to
be the case if the notion of species pertained to [the nature of] man on account of the being
35 “ For it is not necessary that if this is a man and that is a man, then they should have the same
humanity, just there is no numerically one whiteness in two white things; rather, all that is needed is
that one should be similar to the other in that this one has a humanity just as does the other; whence
the intellect, taking humanity not insofar as it is the humanity of this one, but insofar as it is humanity, forms an intention common to all.” SN 2 d. 17, q. 1, a. 1. Cf. “The individuation of the common
nature in corporeal and material things comes from the corporeal matter contained under determinate
dimensions; whereas a universal is abstracted from this sort of matter and from the individuating material conditions. Therefore, it is clear that a similitude of a thing received in a sense [organ] represents
the thing insofar as it is singular, while received in the intellect, it represents the thing on account of
its universal nature; and this is why the senses cognize singulars, and the intellect universals, and that
the sciences concern the latter.” In De Anima, II, lc. 12. Cf. In Post., II, lc. 20. Cf. G. Klima, “The Medieval
Problem of Universals,” The Stanford Encyclopedia of Philosophy (Winter 2004 edn), ed. Edward N. Zalta,
<http://plato.stanford.edu/archives/win2004/entries/universals-medieval/>.
36 Averroës, In De Anima, 1, com. 8: “one should not believe that the definitions of genera and species
are definitions of general things existing outside the soul; rather, they are definitions of particular things,
but the intellect produces universality in them.”
37 Avicenna, Metaph., 5, 2: “Universality exists only in the soul.”
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it has in Socrates, or according to its absolute consideration, for whatever pertains to a man
insofar as he is a man is predicated of Socrates. Nevertheless, “to be predicated” pertains to
a genus per se, for it is included in its definition.38 For predication is something completed
by the action of the judgment-forming intellect, but having some foundation in reality, namely
the unity of those [things] of which one is predicated of the other. Therefore, the notion of
predicability can be included in the definition of the intention that is the genus, and this is
likewise completed by an act of the intellect. Still, that to which the intellect attributes the
intention of predicability (predicating it of something else), is not the intention of genus, but
rather that to which the intellect attributes the intention of genus, as it is signified by the
name “animal.”39
It is clear, therefore, how essence is related to the notion of species; for the notion of species
is not one of those [attributes] that pertain to it according to its absolute consideration, nor
is it one of those accidents that follow upon it on account of the existence it has outside the
soul, such as whiteness or blackness, but it is one of those accidents that follow upon it on
account of the existence it has in the intellect; and it is also in the same way that the notion
of genus or difference applies to it.
Chapter 5. On the Essence of Immaterial Substances
Now it remains to be seen how essence is found in separate substances, namely, in the soul,
in the intelligences [i.e., angels], and the first cause [i.e., God]. Although everybody acknowledges the simplicity of the first cause, nevertheless, some people want to introduce the
composition of matter and form in the intelligences and the soul. This position originates
from Avicebron, the author of the book called The Fountain of Life. But this contradicts
the common teaching of philosophers, who called them substances separate from matter,
and proved them to be without any matter whatsoever. The most powerful demonstration
of this claim proceeds from their ability to understand.
It is clear that forms are not actually intelligible, unless they are separated from matter
and its [individuating] conditions, and they are not rendered actually intelligible, except by
the power of an intelligent substance, insofar as it receives them and works on them.
Therefore, an intelligent substance has to be immune from matter in every way, so that it
neither has matter as its part, nor does it exist as a form impressed in matter, as is the case
with material forms.
And it cannot be said that intelligibility is impeded not by just any sort of matter, but only
by corporeal matter. For if this feature, namely, that it impedes intelligibility, pertained only
to corporeal matter as such, then matter would have to have this feature from the corporeal form, since matter is called corporeal only insofar as it is informed by the corporeal form.40
But this cannot be the case, because the corporeal form itself is actually intelligible, just as
38 For according to the definition of Porphyry, a genus is something that can be predicated of
several specifically different things in response to the question “What is this?”.
39 That is to say, although the above-mentioned definition of the term “genus” includes that a genus
is predicable, still, this definition does not state it of the intention of genus (expressed by the term “genus”)
that it is predicable, but of that to which the term “genus” (as well as its definition) applies, namely,
to a generic concept, such as the concept expressed by the term “animal.” Cf. ST 1 q. 85, a. 2, ad 2,
included in section 10 of this volume.
40 “ The corporeal form” ( forma corporalis) Aquinas is talking about here is the form of corporeity
[corporeitas], the form signified by the name “body” (corpus).
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any other form that is abstracted from matter. Therefore, in the soul or in the intelligences
there is no composition of matter and form in any way, so that they could be understood
to have matter in the same sense in which corporeal substances do. But there is the composition of form and existence [esse] in them. Whence in the commentary on the ninth
proposition of The Book of Causes it is stated that an intelligence is something having form
and existence; and “form” is taken there for the quiddity or the simple nature itself.
And it is easy to see how this can be the case. For if two things are related in such a way
that one of them is the cause of existence of the other, then the cause can exist without the
other, but not conversely. But this is the relationship between matter and form, because
form gives existence to matter; therefore, matter cannot exist without a form, but it is not
impossible for a form to exist without matter. For a form, insofar as it is a form, does not
have to depend on matter, but if one finds forms that cannot exist without matter, this
happens to be the case because of their distance from the first principle, which is the first,
pure actuality. So, those forms that are the closest to the first principle are forms subsisting
without matter, for the whole genus of forms does not require matter, as has been said, and
forms of this sort are the intelligences; so, the essences or quiddities of these substances do
not have to be other than their form itself.
The difference, therefore, between the essence of a simple substance and that of a composite substance is that the essence of a composite substance is not only the form, but it
comprises both form and matter, whereas the essence of a simple substance is its form only.
And this gives rise to two other differences.
The first is that the essence of a composite substance can be signified either as a whole
or a part, which results from the designation of matter, as has been explained. Therefore,
the essence of a composite thing is not predicated in every way of the composite thing itself,
for we cannot say that a man is his own quiddity.41 But the essence of a simple thing, which
is its form, can only be signified as a whole, for there is nothing there apart from the form,
so there is nothing that could receive the form. Therefore, no matter how we consider the
essence of a simple substance, it is predicated of the simple substance. This is why Avicenna
says that the quiddity of a simple substance is the simple substance itself, for there is nothing else [in the simple substance] to receive it.
The second difference is that the essences of composite things, since they are received
in designated matter, are multiplied by the division of designated matter, whence in their
case it happens that there are numerically distinct things in the same species. However, since
the essence of a simple thing is not received in matter, in their case there cannot be this
kind of multiplication; therefore, in the case of these substances, there cannot be several
individuals in the same species, but there are as many species as there are individuals, as
Avicenna expressly claims.42
However, these substances, although they are only forms without matter, are not simple
in every way, so that they would be pure actuality, but they are mixed with potentiality,
which is clear from the following.
41 The essence of the composite thing is not predicated of it as it is signified exclusively, by means
of an abstract term (say, “humanity”), but it is predicated of the thing as it is signified non-exclusive,
by means of a concrete term (say, “man”). But the essence of a simple thing is predicated of the thing
whether it is signified by a concrete term (say, “angel”) or by an abstract term (say, “angelity”). So,
while it is true that Socrates is a man, it is not true that Socrates is a humanity; but it is true that Gabriel
is an angelity just as well as it is true that Gabriel is an angel. Compare the end of c. 3.
42 Avicenna, Metaph., 5, 5.
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Whatever is not included in the understanding of an essence or quiddity is coming to it
from outside, entering into composition with the essence; for no essence can be understood
without its parts. But every essence can be understood without even thinking about its
existence, for I can understand what a man or a phoenix is, and not know whether it
actually exists in the nature of things.43 Therefore, it is clear that existence is distinct from
essence, unless, perhaps, there is a thing whose quiddity is its own existence. And this thing
would have to be unique and the first [being]. For something can only be multiplied either
by the addition of some difference, as the nature of the genus is multiplied in the species, or
on account of the reception of a form in diverse pieces of matter, as the nature of the species
is multiplied in several individuals, or on account of one of the things multiplied being
separate and the other received in something; for example, if there were some separate
warmth, then it would be separate from a non-separate warmth [i.e., the warmth of a warm
body] on account of its separation itself. However, if there were a thing that is existence
only, so that it would be subsistent existence itself [ipsum esse subsistens], then this existence
would not receive the addition of a difference, for then it would no longer be existence only,
but existence and some form besides; and even less would it be receptive of the addition of
matter, for then it would already be not subsistent, but material existence. Therefore, it remains
that there can only be one thing that is its own existence. And so, in any other thing, the
existence of the thing has to be other than its quiddity or nature or form. Therefore,
the intelligences have to have existence besides their form, which is why it was said that an
intelligence is form and existence.
Now, everything that a thing has44 is either caused in it by its own principles, as the
ability to laugh in man, or it comes to the thing from an external source, as the light in
the air is coming from the sun. But the existence of a thing cannot be caused by its form or
quiddity itself (I mean, as by an efficient cause), for then a thing would be its own cause,
and would bring itself into existence, which is impossible. Therefore, all such things,
namely, those that have their existence as something distinct from their nature, have to have
their existence from something else. However, since everything that is through something
else [per aliud] is reduced to what is through itself [per se] as its first cause, there has to be
something that is the cause of existence for everything, since it is existence only. For otherwise the series of causes would go to infinity, since every thing that is not existence only
has a cause for its existence, as has been said. It is clear, therefore, that an intelligence is
both form and existence, and that it has its existence from the first being that is existence
only; and this is the first cause, which is God.
43 Despite modern presumptions to the contrary, Aquinas did not take a phoenix to be a merely
fictitious, mythical bird by definition. Cf. G. Klima, “On Kenny on Aquinas on Being: A Critical Review
of Aquinas on Being by Anthony Kenny,” feature review in International Philosophical Quarterly, 44 (2004),
pp. 567– 80. So, for Aquinas, the phoenix is rather the illustration of a real, yet ephemeral natural
phenomenon, much like a lunar eclipse or a rainbow, of which we may have the scientific definition
(and thus we may know perfectly well its real essence, as opposed to merely knowing the meaning of
its name), and yet we may not know whether its real essence is presently actualized in any actually
existing individual.
44 “Everything that a thing has” is implicitly contrasted with “what a thing is,” i.e., what a thing is
by its essence. For of course what a thing is by its essence is not caused in the thing either by intrinsic
or by extrinsic principles. This is the reason why God, who is existence, is absolutely uncaused, whereas
everything that merely has existence has to have it caused by something.
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Everything that receives something from something else is in potentiality with respect
to what it receives and the latter is its actuality. Therefore, the quiddity or form that is the
intelligence has to be in potentiality with respect to the existence it receives from God, while
the existence received is its actuality. And in this way there is potentiality and actuality in
the intelligences, although not form and matter, unless equivocally. Thus, also to undergo
[change], to receive, to be a subject, and the like, which appear to pertain to things on account
of their matter, equivocally apply to intellectual substances and to corporeal ones, as the
Commentator remarks in Book 3 of On the Soul.45 And since, as has been said, the quiddity
of an intelligence is the intelligence itself, its quiddity or essence is what itself is, and its
existence received from God is that by which it subsists in the nature of things. For this
reason, these substances are said to be composed of that from which something is [ex quo est] and
that which is [quod est], or that which exists [id quod est] and existence [esse], as Boethius declares.46
And since intelligences have potentiality and actuality, it is not difficult to see how there
can be a multitude of intelligences, which would be impossible if they had no potentiality
whatsoever. This is why the Commentator says in his commentary on Book 3 of On the
Soul that if we did not know the nature of the possible intellect, then we could not discover the multitude of intelligences.47 So, they are distinguished by the different degrees of
potentiality and actuality, so that a superior intelligence, which is closer to the first, has more
actuality and less potentiality, and so on for the rest. And this series is completed in the human
soul, which is at the lowest grade among intellectual substances. Therefore, the possible
intellect is related to intelligible forms as prime matter (which is at the lowest grade among
sensible beings) is related to sensible forms, as the Commentator remarks in his commentary on Book 3 of On the Soul.48 And for this reason the Philosopher compares it to a
clean slate on which nothing is written.49 Therefore, since among other intellectual substances
it has the greatest amount of potentiality, it is so close to material substances that it attracts
a material thing to share its existence, so that from the soul and the body there results a
single existence in one composite thing; although that existence, insofar as it is the existence
of the soul, is not dependent on the body.50 Therefore, after this form, which is the soul, we
find other forms that have even more of potentiality and are closer to matter, so that they
have no existence without matter. But even among these we find some order and degrees
down to the forms of elements that are the closest to matter. So these do not even have any
operation except what is demanded by the active and passive qualities and those that
dispose matter to receive form.51
45 Averroës, In De Anima, 3, com. 14.
46 Boethius, De hebdom. ML 64, col. 1311 C.
47 Averroës, In De Anima, 3, com. 5.
48 Averroës, In De Anima, 3, com. 5.
49 Aristotle, De Anima, 3, c. 4, 429b31.
50 Besides his thesis of the unity of substantial forms, this is Aquinas’ most fundamental metaphysical rationale for his uncompromising position on the substantial unity of body and soul, markedly
distinguishing his philosophical anthropology from all forms of dualism both before him (Plato,
Augustine, medieval Augustinians) and after him (Descartes, and post-Cartesian philosophy up to the
present day), as well as from materialism.
51 This is an allusion to Aristotle’s doctrine in his “On Coming to Be and Passing Away” (De Generatione
et Corruptione), according to which each of the four elements is characterized by a pair of active and
passive qualities (active: hot, cold; passive: wet, dry), which account for the material dispositions of all
natural bodies, based on the various proportions of the elements that make them up.
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Chapter 6. An Overview of the Essences of Substances in Relation to
Logical Intentions
Having seen these points, it should be clear how essence is found in various things. Among
substances, there are three ways of having their essence: for there is something, namely, God,
whose essence is his own existence; and for this reason there are philosophers who say that
God does not have a quiddity or essence, because his essence is not something other than
his existence. And from this it follows that he is not in a genus. For everything that is in a
genus has to have its essence besides its existence, since the quiddity or nature of their genus
or species is not distinguished on account of the nature of those things of which these are
the genus or species, but existence is diverse in the diverse things.52
Nevertheless, if we say that God is existence only, we still do not have to fall into the
error of those who stated that God is that universal existence whereby each and every
thing formally exists.53 For the existence that is God is of such a condition that nothing can
be added to it; therefore, it is distinct from every other existence by its own purity itself.54
And for this reason, in the commentary of the ninth proposition of The Book of Causes it
is stated that the first cause, which is existence only, is individuated through its own pure
52 This argument is stated in a very condensed form in the text, so it needs some explication. The
idea is that if there are several things in the same genus or species, then they are obviously not distinguished by their generic or specific nature, since they belong to the same genus or species precisely
on account of sharing the same generic or specific nature. Therefore, since they are nevertheless distinct, they have to be distinct on account of their distinct acts of existence (which in turn are distinct,
of course, on account of the relevant principles of specification and individuation, namely, on account
of the specific differences of different species in the same genus, and the distinct parcels of designated
matter of the different individuals in the same species). So, if the generic or specific nature of several
things in the same genus or species is not the reason why they are distinct, but their existence is, then
the generic or specific nature of these things must be distinct from their existence. That is to say, if
something is in a genus or a species, then its existence must be distinct from its essence, whence if the
existence of something is not distinct from its essence, then it is not in a genus or a species. Therefore,
divine nature cannot fall under any specific or generic concept, which means that divine nature cannot have a quidditative definition (which would have to contain at least a generic concept); all we can
have is several partial characterizations of divine essence, which, however, is ultimately inexhaustible
and incomprehensible by means of our finite concepts. Cf. ST 1 q. 3, a. 5, the third argument in the
body of the article.
53 This was the pantheistic error of the Amalricians (Lat. Almarici, Almariani, Amauriani), the followers
of Amalric of Bena (in French, Amaury de Bène or de Chartres, in Latin, Almaricus, Amalricus,
Amauricus), a professor at the University of Paris, who died between 1204 and 1207. The Amalricians
managed to establish a secret sect (allegedly devoted to ritualistic orgies, in the name of everything being
divine), which was discovered and eradicated by the authorities in 1210. Their leaders, after refusing
to recant, were burned at the stake, and their teachings were officially condemned (in 1210 in Paris,
when Amalric’s bones were exhumed and buried in non-consecrated ground, and again in 1215, at the
Fourth Lateran Council).
54 In Roland-Gosselin’s edition – “Le De ente et essentia de S. Thomas d’Aquin. Texte établi d’après
les manuscrits parisiens. Introduction, notes et études historiques,” ed. M. D. Roland-Gosselin,
Bibliothèque thomiste, 8, J. Vrin (Paris: Le Saulchoir/Kain, 1926) – this sentence is completed by the
following: sicut si esset quidam color separatus, ex ipsa sua separatione esset aliud a colore non separato – “just
as if there were a separate color, then it would be distinct from a non-separate color by its very separation.” But Boyer finds this addition inauthentic.
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goodness. The common existence [esse commune],55 however, just as it does not include in
its understanding the addition of anything, does not exclude from its understanding any
addition; for otherwise, [if such an addition were excluded from its understanding, then]
no single thing in which something is added to the thing’s existence could be understood
to exist.
Likewise, although God is existence only, this does not mean that he should lack other
perfections or excellences; rather, he has all perfections of all kinds, and for this reason he
is said to be perfect absolutely speaking,56 as the Philosopher and the Commentator assert
in Book 5 of the Metaphysics.57 But he has all these perfections in a more excellent manner
than any other thing, for in him they are one, and in others they are diverse. And the reason
for this is that he has all these perfections on account of his simple existence; just as, if there
were someone who on account of a simple quality could perform the operations of all
qualities, then in that simple quality he would have all qualities, so too does God have
all perfections in his existence itself.58
In the second way, essence is found in created intellectual substances, in which existence
is other than essence, although that essence is there without matter. Therefore, their
existence is not subsistent, but received [in their essence], and so it is finite and delimited to
the capacity of the receiving nature. But their nature or quiddity is subsistent, not received
in some matter. And this is why it is stated in The Book of Causes59 that the intelligences are
infinite from below and finite from above; for they are finite with regard to their existence
that they receive from above, but they are not finite from below, for their forms are not
delimited to the capacity of some matter receiving them. And for this reason in their case
there is no multitude of individuals in the same species, as has been declared, except in
the case of the human soul, on account of the body with which it is united. And although
its individuation is temporarily dependent on the body with regard to its beginning (for
it cannot acquire an individuated act of existence, except in the body that it makes actual),
nevertheless, it does not have to lose its individuation with the removal of the body. For
once it has separate existence (after it has acquired individuated existence because it was made
the form of this body), that existence always remains individuated.60 And this is why
55 esse commune – existence as conceived by means of our common concept of existence.
56 As opposed to being a perfect this and a not-so-perfect that, as we can say of a man that he is, for
example, a perfect lover but a not-so-perfect husband, or vice versa.
57 Aristotle, Metaph., 5, c. 16, 1021b30; Averroës, com. 21; Commentary of St. Thomas, lc. 21.
58 Another (necessarily imperfect) analogy might be that just as he who has a Swiss army knife has
all the tools of a toolbox in his pocket, so does God have all perfections in his existence.
59 Liber de Causis, lc. 16. Cf. lc. 4 and 5.
60 Obviously, if there are two souls already possessing their individual acts of existence on account
of the fact that one of them is the form of this body and the other is the form of that body, then, if
they continue existing after losing the bodies they actually inform, they do not have to merge into one
soul. Indeed, merging into one would be precisely the loss of their individual existence, contrary to
the assumption that each individually survives the loss of its body. To be sure, this is an assumption
Aquinas does not prove here, he merely answers a possible objection on the basis of this assumption.
But he does provide serious metaphysical arguments for the immateriality and the consequent immortality of the soul elsewhere, for example, in his Questions on the Soul (Quaestio Disputata de Anima),
qq. 2 and 14. Cf. SN 3, d. 5, q. 3, a. 2, d. 22, q. 1, a. 1; SN 2, d. 19, a. 1, SN 4, d. 50, q. 1, a. 1; Questiones
Quodlibetales (QDL) X, q. 3, a. 2; Summa contra Gentiles (ScG), II. 79–81; ST 1, q. 75, a. 4, a. 6; Comp.
Theol., c. 84. For a detailed discussion of Aquinas’ arguments for the immateriality of the soul, see
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Avicenna says that the individuation and multiplication of souls are dependent on the body,
as far as their beginning is concerned, but not with regard to their end.61 And since in these
substances their quiddity is not the same as their existence, they can be sorted into categories;
whence they have genera, species, and differences, although their proper differences are
hidden from us. For their essential differences are unknown to us even in the case of
sensible substances, whence we signify those through accidental differences that stem from
the essential ones, as a cause is signified through its effect, as when being bipedal is taken to
be the difference of man. But the proper accidents of immaterial substances are also
unknown to us; therefore, we cannot signify their [essential] differences either in themselves
or through accidental differences.
We should know, nevertheless, that genus and difference cannot be taken in the same
way in those substances and in sensible substances. For in the case of sensible substances,
their genus is taken from what is material in the thing, whereas the difference is taken from
what is formal in it; this is why Avicenna says in the beginning of his book On the Soul that
among things composed of matter and form, the form “is a simple difference of that which
it constitutes.”62 But not in the sense that the form is the difference itself, but in the sense
that it is the principle of the difference, as he also says in his Metaphysics;63 and this sort of
difference is said to be simple difference, for it is taken from what is a part of the quiddity,
namely, from the form. Since, however, immaterial substances are simple quiddities, in their
case the difference cannot be taken from a part of the quiddity, but [it has to be taken] from
the whole quiddity. And this is why Avicenna remarks at the beginning of his book On the
Soul that a simple difference “pertains only to those species whose essences are composed
of matter and form.”64
Likewise, their genus is taken from their whole essence, but in a different manner. For
one separate substance agrees with another in immateriality; but they differ in their grade
of perfection, in accordance with their being farther away from potentiality and being closer
to pure actuality. Therefore, their genus is taken from that which pertains to them insofar
as they are immaterial, such as their intellectuality or some such feature; but their difference
is taken from their grade of perfection, which, however, is unknown to us. Yet, these
differences do not have to be accidental, just because they correspond to greater or lesser
perfection, which does not diversify species. For the grade of perfection in receiving
specifically the same form does not diversify the form, as something whiter and something less white [do not differ specifically] on account of participating in the same nature of
whiteness [to different degrees]; but the diverse grades of perfection among the forms
or natures themselves do diversify the species; as nature proceeds stepwise from plants to
animals, among which there are some intermediaries between animals and plants according
to the Philosopher in his On Animals.65 Again, intellectual substances do not have to be classified
G. Klima, “Aquinas’ Proofs of the Immateriality of the Intellect from the Universality of Thought,”
Proceedings of the Society for Medieval Logic and Metaphysics, <http://www.fordham.edu/gsas/phil/
klima/SMLM/PSMLM1.pdf>, 1 (2001), pp. 19–28. (See also Robert Pasnau’s comments and my
rejoinder in the same volume, pp. 29–36 and pp. 37–44, respectively.)
61 Avicenna, De Anima, 5, 3.
62 Ibid., 1, 1.
63 Avicenna, Metaph., 5, 5.
64 Avicenna, De Anima, 1, 1.
65 Aristotle, Hist. Animal., 8, c. 1, 588b4–6.
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in terms of two real differences; for this cannot be done with all things, as the Philosopher
says in Book 11 of his On Animals.66
In the third way, essence is found in substances composed of matter and form, in which
existence is received and finite, both because they have their existence from something
else and because their nature or quiddity is received in designated matter. And so they are
finite both from above and from below; and in their case, because of the division of
designated matter, it is already possible to have several individuals in the same species. And
we have already discussed earlier how the essences of these substances are related to the
logical intentions.
Chapter 7. The Essences of Accidents
Now it remains to be seen how essence is found in accidents; for we have explained how it
is found in all substances. And since, as has been said, an essence is what is signified by a
definition, accidents have to have their essence in the way they have a definition. But they
have an incomplete definition, for they cannot be defined unless their subject is included
in their definition, and this is because they do not have their existence separately from
their subject, but, just as from form and matter there results a substantial act of existence
[esse substantiale] when they are united, so from an accident and its subject there results an
accidental act of existence [esse accidentale], when the accident comes to the subject. And
for this reason even a substantial form does not have a complete essence, (and neither does
matter); for in the definition of a substantial form one has to include that of which it is the
form, and so its definition has to be framed with the addition of something that is outside
its genus, similarly to the definition of an accidental form; therefore, the natural scientist,
who considers the soul only insofar as it is the form of a natural body, includes the natural
body in the definition of the soul.67
But there is an important difference between substantial and accidental forms. For just as
a substantial form does not have its existence separately from that which it informs, neither
does that which it informs, namely, matter. Therefore, their union results in that act of
existence [esse] in which the thing itself subsists, and they yield something that is one per
se.68 Therefore, their union yields a certain essence; and so the form, although considered in
itself not a complete essence, is nevertheless a part of a complete essence. But that which is
informed by an accident is in itself a complete being, subsisting in its own existence, which
66 Aristotle, Part. Animal., 1, c. 2, 642b5 –7.
67 The soul is defined by Aristotle as the first actuality (i.e., the substantial form, as opposed to
an accidental form, which would be a second or secondary actuality) of an organic natural body.
Therefore, since no form is a body and no body is a form (because a body has to be a composite of
matter and form), the definition of the soul includes something that is outside its genus, which means
that this definition contains something that is outside the essence or quiddity of the soul, given that
the quiddity of anything is what is signified by its quidditative definition strictly speaking, consisting
of genus and difference. Accordingly, this “naturalist” definition of the soul is not its quidditative definition
strictly speaking. Perhaps, a strict metaphysical definition could be: the soul is a vivifying substantial
form. But of course this definition would apply even to the living subsisting forms of separate substances (and at least analogically even to God), whence it cannot be regarded as a definition in natural science, which only deals with the material universe.
68 Aquinas is talking here about something that is one per se, say, a man, as opposed to something
that is one per accidens, i.e., by mere coincidence, say, a man who happens to be a teacher. Cf. n. 6 above.
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naturally precedes the supervening accident. Therefore, the supervening accident does not
cause by its union with the thing that act of existence in which the thing subsists, i.e., that
on account of which the thing is a per se being, but it causes a certain secondary act of
existence, without which the subsisting thing can be understood to exist, just as anything
that is primary can be understood without what is secondary.69 Therefore, an accident and
its subject do not yield something that is one per se, but something that is one per accidens
[by coincidence].70 Therefore, from their union there does not result some essence, as does
from the union of matter and form; and for this reason an accident is neither a complete
essence nor a part of a complete essence, but just as it is a being with some qualification
[secundum quid], so too, it only has essence with some qualification.
However, in any genus, that which maximally and most truly exemplifies its kind is the
cause of those that do so to a lesser degree, as fire, which is ultimately hot, is the cause of
heat in all hot things, as stated in Book 2 of the Metaphysics;71 therefore, substance, which is
the first in the kind of all beings, and which maximally and most truly has essence, has to
be the cause of accidents, which participate in the nature of being only secondarily and with
qualification [secundum quid]. But this happens in various ways. Since matter and form are
the parts of substance, some accidents primarily stem from form, others from matter. But
there is a certain form whose existence [esse] is not dependent on matter, as the intellectual
soul, whereas matter only has existence through form. So some accidents stemming from
the form do not communicate with matter, such as thinking, which does not take place in
69 For Aquinas, this is the fundamental difference between substantial and accidental forms: the act
of being of a substantial form is the same as the act of being of the thing informed by it, whereas the
act of being of an accidental form is distinct from the act of being of the thing it informs; therefore, it
is logically impossible for a thing to exist without its substantial form, but it is logically possible for
the thing to exist without its accidental form (even if it may naturally be impossible, as in the case of
separable accidents). It is also worth noting that from this claim it is very easy to prove Aquinas’
thesis of the unity of substantial forms. For suppose a substance s has two distinct substantial forms,
f and g. Since f and g are supposed to be two distinct entities, they have to have two distinct acts
of existence, ef and eg. But if both f and g are supposed to be substantial forms of s, then their acts of
existence would have to be identical with the same act of existence of s, es, which is impossible (because
if ef = es and es = eg, then ef = eg, contrary to our assumption). Therefore, if both f and g are substantial forms of s, then f = g, for any f and g, i.e., s can have at most one substantial form (and if s exists,
then it has to have at least one, whence it has to have exactly one). Also, equivalently, if s has
two forms, one of which is a substantial form, then the other has to be an accidental form, as Aquinas
usually argues.
70 Subject and accident, having two distinct acts of being, are two entities, which, however, coincide, insofar as the act of being of the accident is also the act of being of the subject, although not its
primary but secondary act. For the being of, say, Socrates’ wisdom is nothing but Socrates’ being wise,
since for Socrates’ wisdom to be is for Socrates to be wise (although it is not for Socrates to be, absolutely speaking (simpliciter), which is why the act of being of the wisdom of Socrates is only a secondary
act of being of Socrates, namely, an act of being with qualification (secundum quid)).
71 Aristotle, Metaph., 2, c. 1, 993b24–6. Again, the example is illuminating only with the assumption
of Aristotelian physics, where it is assumed that the heat of hot things other than fire is the result
of participating in the heat of fire on account of having fire in their composition, whereas fire itself
is maximally hot, because it is hot by its own nature and not by participation. But regardless of the
example, the idea is clear: something that has a property essentially, and not by participation, has
this property maximally and most truly and causes this property to be present to a lesser degree in
all those things that have it by participation.
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a corporeal organ,72 as the Philosopher proves in Book 3 of On the Soul.73 There are others
stemming from the form, however, that do communicate with matter, such as sensing, but
no accident stems from matter without communicating with the form.
But among the accidents stemming from matter there is some further diversity. For some
accidents stem from matter in relation to the specific form, as, for example, the masculine
and feminine gender in animals, whose diversity is reduced to matter, as stated in Book 10
of the Metaphysics; therefore, after removing the form of the animal, these accidents do not
stay behind, except equivocally. Others, however, stem from matter in relation to the
generic form; and so, after removing the specific form, these still stay behind in matter; as
the skin color of a black man, which stems from the mixture of elements and not from the
soul, remains in him after death.74
And since each and every [material] thing is individuated by matter, the accidents that
stem from matter are the accidents of the individual, according to which individuals of
the same species differ from each other. But accidents that stem from form are the characteristic attributes [propriae passiones] of the genus or the species; therefore, they can be found
in all individuals that share the nature of genus or species, as for example the ability to laugh
[risibile],75 because laughter stems from some apprehension of the human soul.
We should also know that accidents sometimes are caused by the essential principles
[i.e., matter and form] in perfect actuality, as heat in fire, which is always actually hot, but
sometimes they are caused only as a certain capacity, which is completed by an external agent,
as the transparency of the air, which is completed by an external shining body;76 and in the
case of such accidents, the capacity itself is an inseparable accident, but its completion, which
comes from an external principle that is outside the essence of the thing or does not enter
the constitution of the thing, is separable, such as motion and the like.
72 According to Aquinas, we do not think with our brain. The brain merely supplies highly processed
sensory information for the intellect to form its concepts and do its thinking with those concepts on
its own, relying on the brain merely for this information input, but not for carrying out the thinking
process itself. But this input is needed for proper mental functioning, which is why brain injuries lead
to mental impairment.
73 Aristotle, De Anima, 3, c. 4, 429b4; Commentary of St. Thomas, lc. 7.
74 To be sure, consistently with Aquinas’ thesis of the unity of substantial forms, even these accidents cannot remain numerically the same (since this accident is this accident on account of pertaining
to this substance; so if this substance is gone, then this accident cannot remain either). However, they
can remain in the sense that the new substance resulting from the corruption of another will have
specifically the same, but numerically distinct accidents on account of the disposition of matter of the old
substance, which determines what sort of substance with what kinds of accidents can result in a natural process of the corruption of the old substance. But Aquinas merely contrasted here accidents that
remain merely equivocally (since being male or female can be said of a dead body only equivocally)
with accidents that remain univocally (since being black or white applies in the same sense to a dead
body as well as to a living body).
75 This is the stock example of a specific, yet non-essential, characteristic [proprium], as introduced
by Porphyry. As Aquinas’ subsequent remark makes clear, the “laughter” of chimpanzees does not count
as laughter in the sense intended here, which is the specifically human kind of laughter that is supposed to come forth from a sudden realization of something inherently funny, as in getting the point
of a political joke (which certainly does not happen with chimpanzees).
76 For the air is not actually transparent, i.e., one cannot actually see through it, unless it is actually
illuminated, i.e., light actually passes through it.
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We should know, however, that in the case of accidents, genus, species, and difference
are taken in a different manner from the case of substances. For in the case of substances,
from the substantial form and matter conjoined in one nature, there results something that
is one per se, which properly falls into the category of substance;77 therefore, in the case of
substances, their concrete names, which signify the composite, are properly said to be in this
category as species and genera, as are the names “man” and “animal.” But matter and
form are not in this way in this category, except by reduction, as principles are said to be in
a category. However, from an accident and its subject there does not result something that
is one per se, whence they are not conjoined in one nature to which the intention of genus
or species could be attributed. Therefore, the concrete names of accidents, such as “white
[thing]” [album] or “educated [person]” [musicum], do not fall under the categories as species
and genera, except by reduction, only insofar as [the species and genera of accidents] are
signified by the corresponding abstract names, such as “whiteness” [albedo] and “education”
[musica]. And since accidents are not composed of matter and form, their genus cannot be
taken from their matter and their difference from their form, as is the case with composite
substances, but their first genus has to be taken from their mode of existence [modus essendi],
insofar as “being” is predicated in different, primary and secondary, senses in the ten categories, as for example quantity is said to be [a being] insofar as it is a measure of substance
[i.e., of what is a being in the primary sense], and quality [is said to be a being] insofar as it
is a disposition of substance, and so on, as the Philosopher explained in Book 4 of the
Metaphysics.78
Their differences, on the other hand, are taken from the diversity of the principles causing them. And since its proper attributes are caused by the proper principles of the subject,
in the definition of an accident defined in the abstract form (in the form in which it is properly in a category), in the place of the difference one has to include its subject, as for example, pugness is defined as the curvature of a nose. But it would have to be the other way
around if it were defined in the concrete form. For in that case their subject is included in
their definition as their genus; since then they would be defined similarly to material substances, in the case of which the nature of the genus is taken from their matter, as we say
that a pug is a curved nose.
The case is similar if one accident is the principle of another, as for example actions, passions, and quantities are the principles of relations, and this is the basis of the Philosopher’s
classification of relations in Book 5 of the Metaphysics.79 However, since the proper principles of accidents are not always obvious, sometimes we take the differences of accidents
from their effects, as when we take “concentrative” [congregativum] and “dispersive” [disgregativum] to be the differences of colors, which are caused by the abundance or paucity of
light, causing the various species of colors.80
77 I am following here Roland-Gosselin’s reading, noted by Boyer in his n. 122, according to which
it is the thing that is per se one (and not its nature, as Boyer’s reading would have it), that properly
falls into the category of substance. To be sure, the nature of a substance is also in the category of substance, insofar as it is signified by all substantial predicates of the thing. But those predicates are predicated of the thing, and not of its nature, although they are predicated of the thing on account of its
having this nature signified by these predicates.
78 Aristotle, Metaph., 4, c. 2, 1002b5–7; Commentary of St. Thomas, lc. 1. Cf. also In Meta., Bk. 5, lc. 9.
79 Aristotle, Metaph., 5, c. 15, 1020b26; Commentary of St. Thomas, lc. 17.
80 Cf. Aquinas, Commentary on the Metaphysics, lb. 10, lc. 9, n. 11: “We should take into consideration, however, that concentrative and dispersive of sight are not the true constitutive differences of white
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In this way, therefore, it is clear how essence and existence are in substances and in
accidents, how they are in the composite and simple substances, and how the logical
intentions are found in all, except in the First Principle, which is of infinite simplicity, to
which therefore the notion of genus or species does not apply, and so neither does a
definition because of its simplicity, in which let there be the end and consummation of
this discussion. Amen.
and black, but rather their effects. But in place of differences, we can take their signs, just as substantial forms or differences are sometimes designated by means of accidents. For the dispersion of sight
results from the vehemence of light, the abundance of which causes whiteness. And the concentration
of sight comes from the contrary cause.” The problem with this illustration, of course, is that Aquinas
never tells us what he means by the “concentration” (or “gathering,” “bringing together”) and
“dispersion” (or “scattering,” etc.) of sight. One might think that “dispersion of sight” is something
like the dazzling of sight by some bright white light, and the “concentration of sight,” accordingly, is
whatever the opposite effect of the lack of light is, but we can only guess. However, the theoretical
point is clear: one may indicate unknown essential differences by means of known accidents caused
by these differences, as when Lavoisier, giving Cavendish’s “inflammable air” the name “hydrogen”
(meaning “water-maker”) in 1783, indicated the unknown difference of hydrogen (its atomic number)
by means of its characteristic effect of producing water when burned in air. A similar illustration is
often used by Aquinas with reference to the (mistaken, but to a barefoot medieval monk very plausible) etymology of the name lapis (“rock”) as laedens pedem (“something that hurts the foot”). In this
case also, the unknown difference on account of which a material substance is a rock is indicated by
some well-known (indeed, painfully obvious) effect it has.
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